The operation of Magnetic Bubble Memories requires that a d.c. magnetic bias field be applied to each of a number of magnetic bubble memory chips which comprise elements of the memory. It would be desirable to house the chips in a structure which serves both to provide the uniform biasing magnetic field and to shield the chip from the influence of stray external magnetic fields.
In accordance with the illustrated preferred embodiment, the present invention provides a shielded magnetic bias housing which includes a pair of cup-like structures of a magnetically permeable material fastened together to provide a substantially completely enclosed shielded housing for a magnetic bubble chip. A flat disc of a magnetically hard material is interiorly affixed to the bottom of each of the cup-like elements. The magnetic discs are magnetized parallel to the normal to the surface of the discs to provide a bias magnetic field in the gap between the discs.
To magnetize the discs, the permeable housing structure including the magnetic discs is subjected to a strong external magnetic field which saturates the magnetization of the permeable housing and strongly magnetizes the magnetic discs to a value greater than the ultimate value desired for the bias magnetic field. The housing and magnetic discs are then subjected to a second external field in a direction opposite that of the first applied field which saturates the housing in the opposite direction and reduces the magnetization of the magnetic discs to a precise value desired for the bias field. It has been found that for various choices of magnetic materials, the final value of the bias field varies linearly with the strength of the latter applied magnetic field over a wide range of values of the applied field. Thus if the bias field is pre-adjusted in accordance with the abovedescribed method, a precise and reproducible value of bias field may be obtained without the use of additional trimming screws or other mechanical devices in the housing to shunt magnetic flux or vary the distance between the discs.